Is it possible that people who have been exposed to less virulent strains of virus that are similar to, eg, Covid-19, may be more immune to it than those who haven't been exposed?
If so, can we define a concept like "immunosimilarity" to quantify this effect?
The summary says it all. But to give an example, imagine a mild virus that has gone the rounds of Region A. Later it makes its way to Region B, and immediately mutates into a more rapidly spreading strain that puts B's inhabitants at great risk. Now that new strain travels back to Region A. Question is, could it be that the new strain would find it harder to get a foothold in Region A, simply because its milder ancestors have triggered people's immune systems there already?